The function of traffic signalling lights is to project a beam of light at specific angles through an aperture of predetermined cross sectional area. Endeavouring to achieve this end, an internal light source provides illumination to a suitable lens. To facilitate more efficient illumination a reflective surface, usually of parabolic shape, is employed to re-direct toward the lens light from the internal source which would otherwise be wasted. The combined action of the reflective surface and reflection from the filament and envelope of the internal source also causes light from external sources, such as the sun, to be re-directed toward the lens, which can lead an observer to believe that the light is on, when in fact the internal source is off. Such an occurrence is known as a "phantom signal".